Underground electrical and pvc boxes...

slow_eddieMarch 30, 2009

Hello all. I'm an adventerous homeowner working on a project to supply my back yard with power. This is my biggest project yet, the most complicated thing I've done to date was install ceiling fans, but at work I deal with electricity all the time.

My plan is to run 3 separate GFCI breakers protecting 20 amp circuits with 12 gauge THHN wire through 3/4 inch pvc conduit.

Circuit 1 is for electrical outlets, one at the side of the house at the beginning of the run and two in the rear on separate sides of my back yard, total run about 140 feet.

Circuit 2 is for my soon to be built outdoor BBQ. It will power the little outdoor fridge as well as a few lights over the BBQ. Total run about 100 feet.

Circuit 3 will provide my patio with 3 electrical outlets as well as power two wet rated ceiling fans and light fixtures above my patio. Total run about 100 feet.

So now my questions.... I haven't talked to the electrical inspector yet and I don't want to seem completely clueless when I do (I want to disguise it as best I can).

I want to run all 3 circuits through one run of 3/4 inch pvc conduit and branch off where convenient for the other circuits. Is it against code for circuits to pass through receptacle boxes used by other circuits, even if there are no splices besides the receptacle circuit?

For example, circuit 1 will have a receptacle about 3 feet under my breaker panel at the start of the run, yet I also want to run the other 2 circuits through that box rather than run separate conduit for each circuit. There will be no splices made in the box for circuits 2 and 3, they will just pass through. Is this ok?

Also, due to the length of the cable run I wanted to use L bodies and T's as pull boxes. I know that you can't bury splices and they need to be accessible, but what if there are no splices and are just used to pull the wire and then continue on unspliced? Are they ok to bury? I noticed they have foam gaskets for weatherproofing. If I can't use L bodies I'm not sure what to use. I didn't see any regular T's at the electrical supply store.

Also, where my receptacles pop up out of the ground I wanted to use FS single gang boxes with a single hole in the bottom of the box. Is it ok to run the 3 conductors in and out of the box through the single hole? Or does their need to be one entry and one exit point? I would hate to have to use the boxes with two holes at the bottom. That would really take away from the sharpness of the install.

Thats it for now, I appreciate all replies.


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Ron Natalie

You need THWN (though most THHN out there is marked THHN/THWN) or something else approved for wet areas.

You can pass wires through other boxes, as long as you don't exceed the box fill rules.

Boxes without junctions in them do not need to be accessible.

6 #12's should fit through the hole in the boxes just fine. Remember that you must use the in-use covers on your receptaclse. My electrician used in-use covers on the switches to, but I thought that was a bit silly.

    Bookmark   March 30, 2009 at 4:03PM
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I'm curious how you are going to pull the wire into and out of the fs box with just one conduit entering it? If you are thinking about burying the T-condulet below the box that would render it inaccessible and thus illegal, according to NEC 314.29. Sorry. How will you access it if it is buried 18in. deep (NEC 300.5), in order to pull the wire anyway? You don't want to have to dig up the yard to re-pull a damaged wire or add another one in the future. You need the two hole fs boxes. They are not that bad looking. They will serve to better support the box as well. Then just use 90s to go both directions. Also, because you are using gfci breakers, you need to pull a neutral with each circuit conductor. 3 circuits, 3 neutrals, 1 ground. The length of the pull should be no problem with a fish tape and some wire lube. Good luck. Hope this helps.

    Bookmark   March 30, 2009 at 4:58PM
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Thanks for the help guys. I'm not really sure on the box fill rules so I will do a google search. All together there will be 9 wires running through one of the receptacle boxes on their way to their separate circuit branches, but one of those circuits will be spliced into the receptacle, while the other 6 wires will just pass through. I'm running a hot, a neutral, and a ground for each circuit (I realize I can get away with one ground wire rather than 3 somehow but I'm not there with my knowledge yet).

12 gauge is 2.25, ground counts as one 2.25, then there is something about receptacles counting as two volumes that I dont quite understand. My best guess would be 20.25. 6 wires, the ground, and the receptacle. Is this even close? Will this be ok in a standard single gang FS box? Do the box fill rules only come into play with spliced wires or do the pass through conductors count as well?


I must be missing something about getting wire in and out of a single conduit going to the receptacle box. I was going to use a T body in the ground about 5 feet from the receptacle box, and just pull the wire into the box one way, splice it in, and then pull it back out on the way to the next receptacle box. And no, I won't splice anything in the T body in the ground, just pass through. No buried splices at all any where, I get that rule.

Also, do the THWN wires need to be clamped inside the pvc boxes? I couldn't find a fitting for this, and I also didn't see any screw holes or anything in the boxes to accommadate a clamp. It just looks like the conduit mates directly up to the bottom of the box.

Thanks for the help, you guys have been great. I appreciate your patience. I use to be scared to death of electrical wires after being shocked as a kid plugging in the Christmas tree (Sad huh?). This is a conquer your fears thing for me.


    Bookmark   March 30, 2009 at 11:44PM
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Eddie, re-read btharmy's post. Ron is mistaken. Conduit bodies must remain accessible - regardless of whether or not they contain splices.

Use 2 conduits, in and out of the FS boxes. You're not going to impress the inspector with a "sharp" looking installation that ignores code. He will already be rolling his eyes at the 3 ground wires.

BTW, you cannot pull 3 grounds through a box and only count one as box fill. You must count all 3.

    Bookmark   March 31, 2009 at 6:36AM
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Ok, so I can't bury conduit bodies, I'll have to find another option. I didn't see any T fittings that were sealed at the electrical shop. So what is the right thing to use underground if you need a T? Maybe someone can link to a photo of something. I guess if it is that much of an issue I can run the circuits in separate conduit. I didn't want to do that because the conduit will be visible going into the ground on the side of the house. One run looks neater than two, but I can live with it if need be.

2 conduits in and out of boxes. If thats what the code says that is what I'll do.

I also revised my plan. 3 circuits was just too much hassle with box fills. I'm going to go with only two circuits. 1 for my patio receptacles and ceiling fans/lights and 1 for my yard receptacles and BBQ area.

I'm also committed to learning how to run only one ground if that is the right way to do it.



    Bookmark   March 31, 2009 at 12:55PM
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As for the T-fittings beintg buried, think about it for a minute. There is not such a thing as a t-fitting without a cover that you can bury, that would make it all but impossible to push a fish tape from one end and get it through the fitting at a 90deg angle up the "t" then back down and another 90deg angle to continue on to the next fixture, not to mention the friction of the wire on the "edge" on the inside of the fitting. As far as the grounds go, 1 ground wire connected to each device will suffice, that's all you need, just make a joint in each device box and connect the device with a pigtail to the joint of the ground conductor.

    Bookmark   March 31, 2009 at 4:13PM
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My question is, are all those wires going to fit in 3/4" conduit? Would larger be better (and run a pull string through too, for future use) and probably not much more expensive? I suppose you are somewhat limited by what can enter the above-ground boxes.

    Bookmark   April 1, 2009 at 2:29AM
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Thanks for all the great advice. I'm going back to the drawing board with my plan. It seems like running two separate runs of conduit with two separate circuits is the way to go. This will avoid the T issue all together and will make pulling the wire easier as well.

Is there anything wrong with light fixtures and receptacles on the same circuit? I was told by a friend of mine in the business that it is better to have lights and receptacles on separate circuits, but they could coexist if need be.

Thanks. I'll post a diagram when I'm done.


    Bookmark   April 1, 2009 at 12:09PM
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In Canada at least, it's not prohibited to mix lighting and outlets. Your outdoor outlets will need to be GFCI protected; your code may allow you to use GFCI outlets instead of a breaker, but it might be more economical to use a GFCI breaker, and there is absolutely NOTHING wrong with having lighting on a GFCI.

    Bookmark   April 1, 2009 at 2:07PM
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Thanks PJB. I'm going with GFCI breakers.


    Bookmark   April 1, 2009 at 8:48PM
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